Magnetic field sensing using magnetoresistive random access memory (mram) cells

ABSTRACT

A magnetic field sensing system includes one or more magnetoresistive random access memory (MRAM) cells, and may be configured to determine one or more of a presence, a magnitude, and a polarity of an external magnetic field incident upon an MRAM cell. In some examples, a control module of the system controls a write current source, or another device, to provide a write current through a write line associated with the MRAM cell to induce a magnetic field proximate to the MRAM cell. The magnetic field may be less than a magnetic switching threshold of the MRAM cell. After initiating the provision of the write current through the write line, the control module may determine a magnetic state of the MRAM cell, and determine a presence of an external magnetic field incident upon the MRAM cell based at least in part on the magnetic state of the MRAM cell.

TECHNICAL FIELD

The disclosure relates to sensors, and, more particularly, to magneticfield sensors.

BACKGROUND

Magnetic field sensors may be used in a variety of applications thatinclude determining one or more magnetic field properties, such as apresence of a magnetic field. For example, such applications may includeelectrical current sensing, object position sensing, navigation, as wellas a number of other applications in which it may be desirable todetermine one or more properties of magnetic fields. In some proposedtechniques, magnetic field sensors are implemented using one or moremagnetoresistive (MR) sensing elements, Hall-Effect sensing elements, ora combination of both.

SUMMARY

In general, this disclosure is directed to techniques, devices, andsystems for sensing magnetic fields using one or more magnetoresistiverandom access memory (MRAM) cells. In some examples described herein, anMRAM cell may be used to determine one or more properties (also referredto herein as characteristics) of a magnetic field incident upon the MRAMcell. In one example, a magnitude of a write current of an MRAM cell isvaried, such that a magnitude of a magnetic field induced by the writecurrent proximate to the MRAM cell is less than a magnetic switchingthreshold of the MRAM cell. In this example, an external magnetic fieldincident upon the MRAM cell and having a magnitude which, in conjunctionwith the magnitude of the magnetic field induced by the write current,is greater than or equal to the magnetic switching threshold of the MRAMcell, may cause the MRAM cell to change its magnetic state. As a result,a presence of the external magnetic field having such a magnitude may bedetermined by detecting the change in the magnetic state of the MRAMcell. In another example, a magnitude and a polarity of the externalmagnetic field also may be determined using the magnitude of the writecurrent, and a polarity of the write current, respectively.

In one aspect, this disclosure is directed to a magnetic field sensingsystem. The system includes an MRAM cell comprising a magnetic switchingthreshold, and a write line associated with the MRAM cell and configuredto conduct a write current so as to induce a write magnetic fieldproximate to the MRAM cell. In the system, the write magnetic field hasa magnitude that is less than the magnetic switching threshold of theMRAM cell. The system further includes a switching element electricallyconnected in series with the write line and configured to provide thewrite current from a write current source. In some examples, themagnetic field sensing system further comprises the write current sourceelectrically coupled to the write line.

In another aspect, this disclosure is directed to a method of sensing amagnetic field. The method includes providing a write current through awrite line associated with an MRAM cell so as to induce a write magneticfield proximate to the MRAM cell. In the method, the write magneticfield has a magnitude that is less than a magnetic switching thresholdof the MRAM cell. The method further includes determining a magneticstate of the MRAM cell after initiating the provision of the writecurrent through the write line. The method still further includesdetermining a presence of an external magnetic field incident upon theMRAM cell based at least in part on the magnetic state of the MRAM cell.

In another aspect, this disclosure is directed to a magnetic fieldsensing device. The device includes means for providing a write currentthrough a write line associated with an MRAM cell so as to induce awrite magnetic field proximate to the MRAM cell. In the device, thewrite magnetic field has a magnitude that is less than a magneticswitching threshold of the MRAM cell. The device further includes meansfor determining a magnetic state of the MRAM cell after initiating theprovision of the write current through the write line. The device stillfurther includes means for determining a presence of an externalmagnetic field incident upon the MRAM cell based at least in part on themagnetic state of the MRAM cell.

The details of one or more examples are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a functional block diagram that illustrates one example of amagnetic field sensing system that includes one or more MRAM cells.

FIGS. 2A-2C are conceptual diagrams that illustrate cross-sectionalviews of one example of an MRAM cell that may be included within themagnetic field sensing system of FIG. 1, and components thereof.

FIG. 3 is a conceptual diagram that illustrates one example of themagnetic field sensing system of FIG. 1 that includes a plurality ofMRAM cells.

FIGS. 4-10 are flow diagrams that illustrate example methods of sensinga magnetic field and determining various characteristics thereof using amagnetic field sensing system that includes one or more MRAM cells.

DETAILED DESCRIPTION

A magnetic field sensing system may be used to detect the presence of amagnetic field, such as an external magnetic field, which is generatedby a source that is external to the magnetic field sensing system. Themagnetic field sensing system may be used to, for example, detect,measure, and retain magnetic field levels. Example applications ofmagnetic field sensing include, but are not limited to, detecting andbuffering external magnetic field disturbances to a device or system(e.g., disturbances from stray magnetic fields, or from purposefullyapplied magnetic fields), and detecting and buffering exposures of thedevice or system to external magnetic fields.

In some examples described herein, a magnetic field sensing systemincludes one or more magnetoresistive random access memory (MRAM) cells.Each MRAM cell may have a magnetic switching threshold, which may be thethreshold magnetic field magnitude level at which the MRAM cell changesmagnetic state. The magnetic field sensing system may determine, usingan MRAM cell, one or more properties of a magnetic field incident uponthe MRAM cell. For example, as described in further detail below, amagnetic state of an MRAM cell of the system may indicate a presence, amagnitude, or a polarity of a magnetic field incident upon the magneticfield sensing system and the MRAM cell. In some examples, the magneticfield sensing system may include an MRAM device that includes multipleMRAM cells.

MRAM refers to a non-volatile memory technology in which data are storedusing magnetic domains. Because MRAM is non-volatile, the data stored inthe magnetic domains are maintained without requiring power to retain orcontinually refresh the magnetic domains. An MRAM cell of an MRAM devicemay respond similarly to a magnetic field that is induced locallyrelative to the MRAM cell, e.g., via write currents provided to a writeline associated with the MRAM cell as part of a write operationperformed in the MRAM device, as well as to external, e.g., stray,magnetic fields. As one example, a particular MRAM cell may respond to amagnetic field induced by a write current (referred to herein as a“write magnetic field”) passing through a write line associated with theMRAM cell to change a magnetic state of the MRAM cell. In this example,a magnitude of the induced write magnetic field may be greater than orequal to a magnetic switching threshold of the MRAM cell so as to changeits magnetic state. The same MRAM cell may respond in a similar manner,i.e., by changing its magnetic state, in response to an externalmagnetic field incident upon the MRAM cell when a magnitude of theexternal magnetic field is greater than or equal to the magneticswitching threshold.

Additionally, a magnetic state of an MRAM cell may change in response toa magnetic field resulting from a combination of an induced writemagnetic field and an external magnetic field when a summed magnitude ofthe write magnetic field and the external magnetic field is greater thanor equal to the magnetic switching threshold of the MRAM cell. Thedevices, systems, and techniques of this disclosure may, in some cases,take advantage of the above-described phenomenon in order to enable theuse of one or more MRAM cells, and MRAM devices, generally, in magneticfield sensing applications, e.g., as magnetic field sensors.

In some examples, devices, systems, and methods for protecting theintegrity of data stored within MRAM systems, for tamper protection, orboth, may implement the magnetic field sensing techniques, devices, andsystems described herein. An MRAM system including a plurality of MRAMcells may include a magnetic field sensing system that includes a subsetof the plurality of MRAM cells, and data storage that includes anothersubset of the plurality of MRAM cells. In some examples, the MRAM systemmay be configured to take a responsive action in response todetermining, using the magnetic field sensing system, that the MRAMsystem has been exposed to external magnetic fields having a particularmagnitude, which may indicate that the data stored by the MRAM systemmay be compromised or otherwise tampered with (e.g., the externalmagnetic field may indicate the interrogation of the MRAM system).

The responsive action may include, for example, generation of anotification (e.g., activating an alarm or another notification device),activation of a locator device (e.g., a GPS device), causing informationstored in the MRAM cells that store data to be erased or becomeunintelligible (e.g., by corrupting the data or causing a decryption keynecessary to read the data in an intelligible form to be deleted). Insome examples, the responsive action may comprise initiating damage ordestruction to one or more components of the MRAM system, such byactivating a destruction device to damage or destroy one or more of theMRAM cells. In some cases, the responsive action may include storinginformation relating to the event, for example, in an MRAM cell of thesystem, where the information may include, for example, the time atwhich the external magnetic field having a particular magnitude wassensed.

Devices and systems described herein may sense magnetic fields, e.g., bydetermining one or more properties of the magnetic fields, using one ormore MRAM cells, including existing and emerging MRAM technologies.Additionally, the techniques described herein may be used to addmagnetic field sensing functionality to MRAM devices used primarily fordata storage, e.g., for purposes of providing diagnostic functionalityand device tamper protection. In this manner, the techniques of thisdisclosure may enable magnetic field sensing devices (e.g., magneticfield sensors) that are implemented using MRAM technologies, as well asprovide for novel and useful applications for sensing magnetic fieldsusing MRAM devices.

FIG. 1 is a functional block diagram that illustrates one example of amagnetic field sensing system that includes one or more MRAM cells. Asshown in FIG. 1, magnetic field sensing system 100 includes an MRAMmodule 102, a control module 104, a positive power supply 110, anegative power supply 112, and a magnetic field input 114. As also shownin FIG. 1, control module 104 is configured to provide one or more writecontrol signal(s) 106 to MRAM module 102, and MRAM module 102 isconfigured to receive magnetic field input 114 and provide one or moreread signals 108 to control module 104. In addition, as also shown inFIG. 1, control module 104 is configured to output one or more controlmodule output signal(s) 116.

System 100 may comprise an electro-mechanical system or device of anysuitable kind, including any combination of mechanical structuralcomponents and hardware, discrete electronic components, digital and/oranalog circuitry and integrated devices, as well as mechanical andelectronic sub-systems or sub-devices of any kind. In the example ofFIG. 1, MRAM module 102 comprises one or more MRAM cells. Also in theexample of FIG. 1, control module 104 is configured to control one ormore components of system 100 (e.g., included within MRAM module 102, orelsewhere within system 100) to, for each of the one or more MRAM cellsincluded within MRAM module 102, provide a write current through a writeline associated with the respective MRAM cell so as to induce a writemagnetic field proximate to the MRAM cell, such that a magnitude of thewrite magnetic field is less than a magnetic switching threshold of theMRAM cell. Examples of control module 104 are described in greaterdetail below. Examples of MRAM module 102 are also described in greaterdetail below, as well as with reference to MRAM cell 202 and MRAM module302 of FIGS. 2 and 3, respectively.

According to the techniques of this disclosure, as one example, system100, including MRAM module 102 and control module 104, may be configuredas a magnetic field sensing system (also referred to as a “magneticfield sensing device” in some examples). In the example of FIG. 1,system 100 may be configured to sense external magnetic fields, such asby determining one or more properties, or characteristics, of magneticfield input 114, e.g., one or more of a presence, a magnitude, and apolarity of magnetic field input 114. System 100, and in particular,MRAM module 102 and control unit 104, may be configured to convertmagnetic field input 114 from an external magnetic field, as describedabove, to one or more electrical signals representative of the externalmagnetic field in order to generate control module output signal(s) 116.In this way, system 100 is configured to sense an external magneticfield that provides magnetic field input 114. For example, controlmodule output signal(s) 116 may comprise one or more voltage and/orcurrent signals indicative of magnetic field input 114, including anycombination of analog and/or digital signals, or other information, usedto represent the one or more properties, or characteristics, of magneticfield input 114 previously described, as well as any other properties orcharacteristics.

As one example, control module output signal(s) 116 may comprise one ormore values indicative of the presence of magnetic field input 114(e.g., a “1” indicating that magnetic field input 114 is present, and a“0” indicating otherwise). As another example, control module outputsignal(s) 116 may comprise one or more values indicative of themagnitude of magnetic field input 114 (e.g., one or more valuesindicating the magnitude in units of amperes per meter (A/m), oersteds(Oe), or other units). As still another example, control module outputsignal(s) 116 may comprise one or more values indicative of a polarityof magnetic field input 114 (e.g., a “0” indicating that magnetic fieldinput 114 has a positive polarity, and a “1” indicating that magneticfield input 114 has a negative polarity, or one or more vectorsindicating the polarity of magnetic field input 114). In addition,control module output signal(s) 116 may represent any combination of thepresence of magnetic field input 114, including a location of magneticfield input 114 relative to one or more MRAM cells included within MRAMmodule 102, the magnitude of magnetic field input 114, and the polarityof magnetic field input 114. In any case, in some examples, in additionto converting magnetic field input 114 from the external magnetic fieldto generate control module output signal(s) 116 used to represent theone or more properties of magnetic field input 114, as described above,control module 104 may further process magnetic field input 114 (e.g.,filter, scale, normalize, level-shift, combine, etc.,) in any manner togenerate control module output signal(s) 116.

In the example shown in FIG. 1, to convert magnetic field input 114 fromthe external magnetic field in order to generate control module outputsignal(s) 116, control module 104 is configured to communicate with MRAMmodule 102 via write control signal(s) 106 and read signal(s) 108, whichare described in greater detail below with reference to FIG. 3. Asdescribed in further detail below, control module 104 may use writecontrol signal(s) 106 to control a write current source, a sink/sourcenode, and/or one or more switching elements of MRAM module 102 toprovide write currents through write lines associated with one or moreMRAM cells of MRAM module 102. As will also be described, control module104 may determine magnetic states of the one or more MRAM cells of MRAMmodule 102 based on read signal(s) 108.

Control module 104 may comprise any suitable arrangement of hardware,software, firmware, or any combination thereof, to perform thetechniques attributed to control module 104 in this disclosure. Forexample, control module 104 may include any of one or moremicroprocessors, microcontrollers, digital signal processors (DSPs),application specific integrated circuits (ASICs), field programmablegate arrays (FPGAs), or any other equivalent integrated or discretelogic circuitry, as well as any combination of such components.Furthermore, control module 104 may include various types of analogcircuitry, in addition to, or in place of, the logic devices andcircuitry described above.

Positive power supply 110 and negative power supply 112 may eachcomprise any power supply unit, module, or circuitry also includedwithin system 100, which may, in some examples, be integrated with MRAMmodule 102 and/or control module 104 within a common enclosure, or on acommon printed board (PB). Although positive power supply 110, negativepower supply 112, MRAM module 102, and control module 104 of system 100are described as separate units or modules for conceptual purposes, insome examples, any combination of these components of system 100 may befunctionally integrated within a common enclosure or housing.

In this disclosure, any reference made to a memory, or a memory device,used to store instructions, data, or other information, may include anyvolatile or non-volatile media, such as random access memory (RAM), readonly memory (ROM), non-volatile RAM (NVRAM), electrically erasableprogrammable ROM (EEPROM), flash memory, magnetic memory (e.g., MRAM),and the like. In some examples, one or more memory devices may beexternal to system 100, MRAM module 102, and/or control module 104, forexample, external to an enclosure or a common PB used to enclose orhouse system 100, MRAM module 102, and/or control module 104. In otherexamples, the one or more memory devices may be internal to system 100,MRAM module 102 (e.g., MRAM cells included within MRAM module 102 forpurposes of storing data), and/or control module 104, e.g., includedwithin a common enclosure or housing, or on a common PB.

System 100 may comprise an MRAM cell (e.g., any of the one or more MRAMcells included within MRAM module 102) comprising a magnetic switchingthreshold. The magnetic switching threshold of the MRAM cell maycorrespond to a minimum magnitude of a magnetic field incident upon theMRAM cell that is required to change a magnetic state of the MRAM cell.For example, the magnetic switching threshold of the MRAM cell may beequal to the minimum magnitude.

In some examples, the magnetic switching threshold of the MRAM cell mayvary, e.g., based on one or more factors, which may be environmental insome examples. For example, the magnetic switching threshold of the MRAMcell may vary based on a temperature of the environment in which theMRAM cell is operating (e.g., over a range of temperature in which MRAMmodule 102 that includes the MRAM cell is designed to be used), time(e.g., with use, or so-called “wear” of the MRAM cell), process (e.g.,relative to a magnetic switching threshold of another MRAM cell, or anexpected magnetic switching threshold, as a result of manufacturing theMRAM cell of this example), or based on any number of other factors orparameters of the MRAM cell and/or its use. In some examples, themagnetic switching threshold of the MRAM cell may vary by about up toabout 5% (e.g., 5% or less) relative to a baseline value of the magneticswitching threshold (e.g., corresponding to a particular operatingenvironment or state of the MRAM cell, or to an expected magneticswitching threshold of the MRAM cell), as a result of changes in one ormore of the factors or parameters described above. As a result of somevariance in the magnetic switching threshold of the MRAM cell, theminimum magnitude of the magnetic field incident upon the MRAM cell thatis required to change the magnetic state of the MRAM cell may also varyby a relatively small percentage (e.g., by about 5% or less).

For this reason, in some examples, the magnetic switching threshold ofthe MRAM cell may be determined or estimated based on the variousfactors or parameters of the MRAM cell and its use described above, forpurposes of performing the techniques of this disclosure. For example,the magnetic switching threshold of the MRAM cell, as described herein,may correspond to a magnetic switching threshold that is determined forthe MRAM cell, or to a magnetic switching threshold that is estimatedfor the MRAM cell, based on one or more of the above-described factorsor parameters of the MRAM cell and its use. Furthermore, the magneticswitching threshold of the MRAM cell may be adjusted as part ofperforming the techniques of this disclosure based any of theabove-described factors or parameters of the MRAM cell and its use,e.g., at any time prior to, during, and following performing thedisclosed techniques.

System 100 may still further comprise a write line (e.g., also includedwithin MRAM module 102) associated with the MRAM cell and configured toconduct a write current so as to induce a write magnetic field proximateto the MRAM cell, wherein the write magnetic field has a magnitude thatis less than the magnetic switching threshold of the MRAM cell. In someexamples, the magnitude of the write magnetic field may be a firstmagnitude, and may be directly proportional (e.g. linearly proportional)to a second magnitude of the write current.

System 100 may further include a write current source electricallycoupled to the write line, and a switching element electricallyconnected in series with the write line and configured to provide thewrite current from the write current source through the write line(e.g., either or both of which may be included within or outside of MRAMmodule 102). Examples of the write current source and the switchingelement, as well as a sink/source node operable in conjunction with thewrite current source, are described below with reference to FIG. 3.

In some examples, control module 104 may be configured to control one ormore of the write current source and the switching element to providethe write current from the write current source through the write line,such that the magnitude of the write magnetic field is less than themagnetic switching threshold of the MRAM cell. In these examples,control module 104 may be further configured to, after initiating theprovision of the write current through the write line, determine amagnetic state of the MRAM cell. Also in these examples, control module104 may be still further configured to determine a presence of anexternal magnetic field incident upon the MRAM cell based at least inpart on the determined magnetic state of the MRAM cell.

In some examples, the magnetic state of the MRAM cell may correspond toa resistance of the MRAM cell. In these examples, to determine themagnetic state of the MRAM cell, control module 104 may be configured todetermine the resistance of the MRAM cell.

In other examples, the magnetic state of the MRAM cell may comprise acurrent magnetic state of the MRAM cell, which may be, for example, themagnetic state of the MRAM cell at the time at which control module 104determines the magnetic state. In these examples, control module 104 maybe further configured to, prior to controlling the one or more of thewrite current source and the switching element to provide the writecurrent, determine an initial magnetic state of the MRAM cell.Subsequently, to determine the current magnetic state of the MRAM cell,control module 104 may be configured to determine a change in theinitial magnetic state of the MRAM cell.

In this manner, magnetic field sensing system 100 of FIG. 1 may beconfigured as a magnetic field sensing system comprising an MRAM cellcomprising a magnetic switching threshold, a write line associated withthe MRAM cell and configured to conduct a write current so as to inducea write magnetic field proximate to the MRAM cell, wherein the writemagnetic field has a magnitude that is less than the magnetic switchingthreshold of the MRAM cell, and a switching element electricallyconnected in series with the write line and configured to provide thewrite current from a write current source.

FIGS. 2A-2C are conceptual diagrams that illustrate cross-sectionalviews of one example of an MRAM cell that may be included withinmagnetic field sensing system 100 of FIG. 1, and components thereof. Asshown in FIG. 2A, MRAM cell 202 is a structure that is configured tostore data (e.g., a single bit of data) magnetically. For example, MRAMcell 202 may include a fixed magnetic layer 226B, a tunnel barrier layer228, and a free magnetic layer 226A. An orientation of a magnetic momentof fixed magnetic layer 226B of MRAM cell 202 is generally fixed for thetemperatures and external magnetic fields in which the MRAM device(e.g., MRAM module 102 of FIG. 1) that includes MRAM cell 202 isdesigned to be used. An orientation of a magnetic moment of freemagnetic layer 226A, however, may be switched (e.g., by writing data toMRAM cell 202) between two states, wherein each state may represent avalue (e.g., a “0” or a “1”) of a single bit of data stored using MRAMcell 202.

As shown in FIG. 2A, MRAM cell 202 may include a first write line 218Aand a second write line 218B, which may be used to write data to MRAMcell 202. First write line 218A extends generally in an x-axisdirection, as depicted in FIG. 2A, while second write line 218B extendsgenerally in a y-axis direction, as also depicted in FIG. 2A. MRAM cell202 also includes a magnetic stack 230, which includes free magneticlayer 226A, tunnel barrier layer 228, and fixed magnetic layer 226B. Insome examples, magnetic stack 230 may be referred to as a magnetictunnel junction (MTJ). In some examples, tunnel barrier layer 228 mayinclude a dielectric, such as an oxide. In some examples, tunnel barrierlayer 228 may include aluminum oxide (Al₂O₃) or magnesium oxide (MgO).

As shown in FIG. 2B, fixed magnetic layer 226B includes a relativelyfixed, or “pinned,” magnetic moment 242, also illustrated in FIG. 2B. Inthe illustrated example, pinned magnetic moment 242 is oriented atapproximately a 45-degree angle relative to first write line 218A andsecond write line 218B (e.g., at approximately a 45-degree anglerelative to both the x-axis and the y-axis in FIG. 2B, where orthogonalx-y-z axes are shown in FIGS. 2A-2C for ease of description). In someexamples, fixed magnetic layer 226B may include a ferromagnetic metal oralloy, such as, for example, nickel (Ni), iron (Fe), or cobalt (Co), oralloys of Ni, Fe, or Co. Example alloys from which fixed magnetic layer226B can be formed may include nickel iron (NiFe), cobalt iron (CoFe),and nickel iron cobalt (NiFeCo). In some examples, fixed magnetic layer226B may be magnetically coupled to an antiferromagnetic layer, whichacts to “pin” magnetic moment 242 of fixed magnetic layer 226B in themanner illustrated in FIG. 2B. The antiferromagnetic layer may includean antiferromagnetic alloy, such as, for example, iron manganese (FeMn),nickel manganese (NiMn), platinum manganese (PtMn), or iridium manganese(IrMn). In some examples, the antiferromagnetic layer may be a bilayeror multilayer, in which the layers have different compositions ormagnetic properties.

In contrast to fixed magnetic layer 226B, free magnetic layer 226Aincludes a “free” magnetic moment that is free to rotate under influenceof a sufficiently strong applied magnetic field, as illustrated in FIG.2C. For example, in a similar manner as described above with referenceto FIG. 1, the sufficiently strong applied magnetic field may have amagnitude that reaches or exceeds a magnetic switching threshold of MRAMcell 202, so as to allow the free magnetic moment to rotate in themanner described above. In some examples, free magnetic layer 226A mayinclude a ferromagnetic metal or alloy, such as, for example, Ni, Fe, orCo, or alloys of Ni, Fe, or Co. Example alloys from which free magneticlayer 226A can be formed may include NiFe, CoFe, and NiFeCo.

For example, at any given time, free magnetic layer 226A may have afirst free magnetic moment 244A or a second free magnetic moment 244B.Free magnetic layer 226A may be switched between first free magneticmoment 244A and second free magnetic moment 244B by the sufficientlystrong magnetic field described above, such as a magnetic fieldgenerated by first write line 218A and/or second write line 218B, or,consistent with the techniques described herein, by an external magneticfield incident upon MRAM cell 202 (e.g., magnetic field input 114 ofFIG. 1).

For example, the magnetic moment of free magnetic layer 226A may beswitched between first free magnetic moment 244A and second freemagnetic moment 244B using write currents (e.g., write current pulses)provided through first write line 218A and second write line 218B. Inparticular, an applied magnetic field (which may be referred to hereinas a “write” magnetic field) may be induced proximate to free magneticlayer 226A by pulses of electrical current flowing through first writeline 218A and second write line 218B. Consider an example in which themagnetic moment of free magnetic layer 226A begins with an orientationthat corresponds to first free magnetic moment 244A, as illustrated inFIG. 2C. Electrical current may be sent through first write line 218A ina direction indicated by arrow 220A (e.g., in the direction of thex-axis of FIG. 2A) and the magnetic moment of free magnetic layer 226Amay rotate to be substantially parallel to arrow 220B. While electricalcurrent still flows through first write line 218A, electrical currentmay be sent through second write line 218B in a direction indicated byarrow 220B (e.g., in the direction of the y-axis of FIG. 2A, out of theplane FIG. 2A), bringing the magnetic moment of free magnetic layer 226Ato a substantially 45 degree angle between arrows 220A and 220B. Currentflow through first write line 218A is then ceased, and the magneticmoment of free magnetic layer 226A rotates to be substantially parallelto the direction of current flow through second write line 218B,indicated by arrow 220B. Finally, current flow through second write line218B is ceased, and the magnetic moment of free magnetic layer 226Arotates to be oriented in a direction that corresponds to second freemagnetic moment 244B. In other examples, electrical current may be sentthrough one or more of first write line 218A and second write line 218Bin any of a variety of directions other than those indicated by arrows220A and 220B.

The orientation of the magnetic moment of free magnetic layer 226A,i.e., one of first and second free magnetic moments 244A, 244B relativeto the orientation of pinned magnetic moment 242 of fixed magnetic layer226B, determines a resistance of magnetic stack 230. For example, theresistance of magnetic stack 230 when pinned magnetic moment 242 and theone of first and second free magnetic moments 244A, 244B are orientedsubstantially anti-parallel (i.e., the magnetic moment of free magneticlayer 226A corresponds to second free magnetic moment 244B) is greaterthan the resistance of magnetic stack 230 when pinned magnetic moment242 and the one of first and second free magnetic moments 244A, 244B areoriented substantially parallel (i.e., the magnetic moment of freemagnetic layer 226A corresponds to first free magnetic moment 244A).

Control module 104 (or another processor) may determine the relativeresistance of magnetic stack 230 (i.e., for each of first and secondfree magnetic moments 244A, 244B of free magnetic layer 226A) bycontrolling the conduction of an electrical current through magneticstack 230, e.g., from top electrode 222 (e.g., coupled to positive powersupply 110 of FIG. 1), through magnetic stack 230, through bottomelectrode 224, and, via transistor 232, to a ground node 236 (e.g.,coupled to negative power supply 112 of FIG. 1). For example, transistor232 may be referenced to ground node 236, as shown in FIG. 2A, and maybe gated at gate terminal 234 using a control signal (e.g., one of readsignal(s) 108 of FIG. 1) to conduct the current through magnetic stack230. Additionally, using an op-amp 238 also shown in FIG. 2A, whichincludes a reference voltage 240 also referenced to ground node 236, thecurrent passing through magnetic stack 230 may be compared to abaseline, or “standard” current.

In the example of FIG. 2A, the current passing through magnetic stack230 generates a voltage between top electrode 222 and bottom electrode224 (i.e., effectively between top electrode 222 and ground node 236)that is proportional to the resistance of magnetic stack 230. Controlmodule 104 may compare this voltage to reference voltage 240, e.g.,using op-amp 238, to generate an output signal, e.g., at output terminal208 of op-amp 238, that is indicative of the resistance of magneticstack 230 (e.g., another one of read signal(s) 108 of FIG. 1). In otherwords, the relative resistance of magnetic stack 230 described abovecomprises the data storage mechanism of MRAM cell 202. For example, ahigh resistance may correspond to a logical state of “1,” while a lowresistance may correspond to a logical state of “0,” of MRAM cell 202.The logical state of MRAM cell 202 may be referred to herein as a“magnetic state” of MRAM cell 202.

Other configurations of MRAM cell 202 may also be used with the magneticfield sensing systems and devices described herein. For example, inother examples, rather than being referenced to ground node 236 having aground potential, as indicated by the “ground” symbol used to depictground node 236 in FIG. 2A, transistor 232 may be coupled to a positivepower supply (not shown) (e.g., positive power supply 110) andconfigured to conduct an electrical current from the positive powersupply, through bottom electrode 224, through magnetic stack 230, to topelectrode 222. In these examples, top electrode 222 may correspond to anegative power supply (e.g., negative power supply 112, or ground node236). Furthermore, in these examples, the input terminal of op-amp 238that is electrically coupled to top electrode 222 as shown in FIG. 2Amay be electrically coupled to bottom electrode 224, or to the positivepower supply itself. In other words, in some examples, an electricalcurrent may flow from top electrode 222, through magnetic stack 230 andbottom electrode 224, to ground node 236 via transistor 232. In otherexamples, however, the current may flow from a positive power supplyelectrically coupled to transistor 232, through bottom electrode 224 andmagnetic stack 230, to top electrode 222 (which may be electricallycoupled to ground node 236) via transistor 232.

Moreover, in still other examples, a ground node or plane of an overallsystem or device that includes MRAM cell 202 may correspond to any offirst write line 218A, top electrode 222, bottom electrode 224, secondwrite line 218B, and ground node 236. In this manner, ground node 236 asshown in FIG. 2A may not necessarily correspond to a ground node orplane of the overall system that includes MRAM cell 202, but rather tonode or plane that has a positive or a negative polarity relative to theground node or plane of the system.

In still other examples, first write line 218A may be electricallycoupled to top electrode 222, thereby electrically coupling first writeline 218A to free magnetic layer 226A of magnetic stack 230 via topelectrode 222. In this manner, in some examples, first write line 218Amay be used to write to MRAM cell 202, as described above, as well as toread from MRAM cell 202 by determining the magnetic state of MRAM cell202, as also described above. In particular, in these examples, firstwrite line 218A may be used to read from MRAM cell 202 using thetechniques described above with reference to determining the relativeresistance of magnetic stack 230.

For example, in cases in which first write line 218A is electricallycoupled to top electrode 222, control module 104 (or another processor)may determine the relative resistance of magnetic stack 230 bycontrolling the conduction of an electrical current from first writeline 218A and top electrode 222 (e.g., jointly coupled to positive powersupply 110), through magnetic stack 230, through bottom electrode 224,and, via transistor 232, to a ground node 236 (e.g., once again coupledto negative power supply 112). In this example, the input terminal ofop-amp 238 that is electrically coupled to top electrode 222 as shown inFIG. 2A may be electrically coupled to any of first write line 218A andtop electrode 222. In this manner, first write line 218A may serve asimilar purpose as top electrode 222 when determining the relativeresistance of magnetic stack 230, and, therefore, the magnetic state ofMRAM cell 202.

The above-described techniques may be used in examples in which MRAMcell 202 is configured to store a single bit of data, e.g., when MRAMcell 202 is configured for a data storage function. In contrast to theabove-described examples in which MRAM cell 202 is configured as a datastorage cell, in examples in which MRAM cell 202 is configured to sensemagnetic field input 114, the magnitude of the magnetic field induced bythe write current passing through each of one or more of first writeline 218A and second write line 218B of FIG. 2A may be less than themagnetic switching threshold of MRAM cell 202, which may be themagnitude of the magnetic field needed to change the magnetic state ofMRAM cell 200, in some examples. In other words, the write currentpassing through each of the one or more of first write line 218A andsecond write line 218B may be controlled using the techniques describedherein to have a sufficiently low magnitude so as to the induce themagnetic field proximate to MRAM cell 202 to have the required (i.e.,generally speaking, lower) magnitude than the magnetic switchingthreshold of MRAM cell 202. That is, in some examples, the writemagnetic field induced by the write current provided through the one ormore of first write line 218A and second write line 218B may be, byitself, insufficient to change the magnetic state of MRAM cell 202(e.g., insufficient to switch the magnetic moment of free magnetic layer226A). MRAM cell 202 may be controlled in this manner using a variety oftechniques, including using a variety of variable write current sources,sink/source devices, and switching elements as described below withreference to FIG. 3, as well as other devices and techniques.

In examples in which MRAM cell 202 is configured to detect an externalmagnetic field, the magnetic moment of free magnetic layer 226A may beswitched between first free magnetic moment 244A and second freemagnetic moment 244B only when the total magnetic field to which freemagnetic layer 226A is exposed is greater than or equal to the magneticswitching threshold of MRAM cell 202 described above. Because theinternal write current, e.g., provided by first write line 218A andsecond write line 218B, induces a write magnetic field that has amagnitude that is less than the magnetic switching threshold, in theseexamples, the total magnetic field refers to the “internal” writemagnetic field, in addition to an external magnetic field.

For example, the magnetic moment of free magnetic layer 226A may beswitched between first free magnetic moment 244A and second freemagnetic moment 244B only when the write magnetic field generated by thewrite current passing through each of one or more of first write line218A and second write line 218B, in conjunction with the externalmagnetic field incident upon free magnetic layer 226A, is greater thanor equal to the magnetic switching threshold of MRAM cell 202 describedabove. In addition, or instead, in some examples, the magnetic moment offree magnetic layer 226A may be switched between first free magneticmoment 244A and second free magnetic moment 244B when the externalmagnetic field alone constitutes the sufficiently strong magnetic fieldhaving a magnitude greater than or equal to the magnetic switchingthreshold of MRAM cell 202. In this manner, in accordance with thetechniques of this disclosure, MRAM cell 202 may be configured to sensemagnetic fields, including determining the one or more properties ofmagnetic fields described herein, rather than being configured to storea single bit of data.

FIG. 3 is a conceptual diagram that illustrates magnetic field sensingsystem 300, which is an example of magnetic field sensing system 100 ofFIG. 1 that includes a plurality of MRAM cells (e.g., a plurality ofMRAM cells 202) and uses one or more of the plurality of MRAM cells tosense an external magnetic field. As shown in FIG. 3, magnetic fieldsensing system 300 includes an MRAM module 302, a control module 304, apositive power supply 310, a negative power supply 312, a magnetic fieldinput 314, one or more control module output signal(s) 316, a writecurrent source 346, and a sink/source node 352. In the example shown inFIG. 3, control module 304 is configured to provide one or more writecontrol signal(s) 306A-306N to a respective one of switching elements348A-348N, and further configured to provide one or more write currentsource control signal(s) 354 to write current source 346. In addition,control module 304 is configured to provide one or more sink/source nodecontrol signal(s) 356 to sink/source node 352. In the example of FIG. 3,write current source control signal(s) 354 and sink/source node controlsignal(s) 356 also may be considered write control signals. Finally,MRAM module 302 is configured to provide one or more read signal(s)308A1-308NM to control module 304.

It should be noted that magnetic field sensing system 300 may be similarto magnetic field sensing system 100 of FIG. 1, as described above. Forexample, MRAM module 302, control module 304, positive power supply 310,negative power supply 312, magnetic field input 314, write controlsignal(s) 306A-306N and write current source control signal(s) 354 andsink/source node control signal(s) 356, read signal(s) 308A1-308NM, andcontrol module output signal(s) 316, may be similar to the respectiveones of MRAM module 102, control module 104, positive power supply 110,negative power supply 112, magnetic field input 114, write controlsignal(s) 106, read signal(s) 108, and control module output signal(s)116 of magnetic field sensing system 100 of FIG. 1. Moreover, writecurrent source 346 and a sink/source node 352 may correspond to, or beincluded within, one or more components of magnetic field sensing system100 of FIG. 1, e.g., within one or more of positive power supply 110,negative power supply 112, MRAM module 102, and control module 104.

In the example of FIG. 3, MRAM module 302 includes a plurality ofswitching elements 348A-348N, a plurality of write lines 318A-318N, anda plurality of MRAM cells 350A1-350NM. As illustrated in FIG. 3, each ofswitching elements 348A-348N may be electrically coupled to a respectiveone of write lines 318A-318N, which may in turn be associated with arespective one or more of MRAM cells 350A1-350NM. For example, as shownin FIG. 3, each of write lines 318A-318N may be associated with arespective one or more of MRAM cells 350A1-350NM, such that, e.g., writeline 318A is associated with MRAM cells 350A1-350AM, write line 318B isassociated with MRAM cells 350B1-350BM, and write line 318N isassociated with MRAM cells 350N1-350NM. In other words, in someexamples, a single one of write lines 318A-318N may be used to conduct awrite current for one or more of MRAM cells 350A1-350AM, as shown inFIG. 3. As described above with reference to MRAM cell 202 of FIG. 2A,each of write lines 318A-318N may correspond to a bit write line or adigit write line of the associated one of MRAM cells 350A1-350NM.

In some examples, MRAM cells 350A1-350NM may be arranged in one or morerows and/or columns, wherein each of write lines 318A-318N correspondsto a particular row or column for a subset of MRAM cells 350A1-350NM.For example, write line 318A may correspond to a row or a column thatincludes MRAM cells 350A1-350AM, write line 318B may correspond to a rowor a column that includes MRAM cells 350B1-350BM, and write line 318Cmay correspond to a row or a column that includes MRAM cells350N1-350NM. In other examples, additional write lines may be present,in addition to write lines 318A-318N depicted in FIG. 3, that correspondto additional rows and columns for MRAM cells 350A1-350NM. For example,an additional write line may be present that corresponds to a row or acolumn that includes MRAM cells 350A1, 350B1, and 350N1. Similarly, anadditional write line may be present that corresponds to a row or acolumn that includes MRAM cells 350AM, 350BM, and 350NM.

Each of switching elements 348A-348N is electrically connected in serieswith a corresponding one of write lines 318A-318N and configured toprovide a write current through the corresponding one of write lines318A-318N from write current source 346 (e.g., via sink/source node352), which is electrically coupled to write lines 318A-318N. Asdepicted in FIG. 3, in some examples, switching elements 348A-348N arelocated between write current source 346 and write lines 318A-318N, suchthat write current source 346 is coupled to write lines 318A-318N via arespective one of switching elements 348A-348N. In other examples,switching elements 348A-348N may be located elsewhere within MRAM module302, e.g., between adjacent ones of MRAM cells 350A1-350NM along thelength of write lines 318A-318N, or between MRAM cells 350A1-350NM andsink/source node 352.

In some examples, each of switching elements 348A-348N may include oneor more of a bipolar junction transistor (BJT)-based switching device, ametal oxide semiconductor field effect transistor (MOSFET)-basedswitching device, or other device or circuitry for providing theswitching functionality described above with reference to switchingelements 348A-348N. In some examples, each of switching elements348A-348N may be configured to merely conduct a current from writecurrent source 346, or from another source (e.g., sink/source node 352),in order to provide the respective write current through thecorresponding one of write lines 318A-318N. In these examples, amagnitude of the current from write current source 346, or from theother source, may be either fixed or variable. For example, controlmodule 304 may vary the magnitude of the current via write currentsource and sink/source node control line(s) 354, 356, as described ingreater detail below. In other examples, each of switching elements348A-348N may itself comprise a current source configured to provide therespective write current through the corresponding one of write lines318A-318N, e.g., by dynamically varying the magnitude of the currentfrom write current source 346, or from the other source, in order toprovide the write current. In this manner, in some examples, switchingelements 348A-348N may be configured to vary a magnitude of the writecurrent provided through each of write lines 318A-318N.

Additionally, in the example of FIG. 3, each of write current source 346and sink/source node 352 may correspond to, or be derived from, one ormore of positive power supply 310 or negative power supply 312. Forexample, write current source 346 may supply (i.e., source) the writecurrent for each of write lines 318A-318N from positive power supply310, and sink/source node 352 may provide a return path for (i.e., sink)the write current to negative power supply 312. As another example,sink/source node 352 may supply the write current for each of writelines 318A-318N from positive power supply 310, and write current source346 may provide the return path for the write current. Furthermore, insome examples, each of write current source 346 and sink/source node 352may be dynamically configured (e.g., by control module 304 via writecurrent source and sink/source node control line(s) 354, 356) to sourceand/or sink the write current for each of write lines 318A-318N betweenpositive power supply 310 and negative power supply 312. In other words,in some examples, each of write current source 346 and sink/source node352 may be dynamically configured to either source or sink the writecurrent, and thereby vary a polarity of the write current, providedthrough each of write lines 318-318N.

In some examples, one or more write current source 346 and sink/sourcenode 352 may be included within each of MRAM module 302, control module302, or within another module of unit of system 300 (e.g., as one ormore high or low-side drivers, current sources, or equivalent circuitryor components, or as power and ground terminals, of MRAM module 302,control module 304, or another module of unit of system 300). Asdiscussed above, in some examples, each of write current source 346 andsink/source node 352 may be integrated, or configured in aninteroperable manner, with one or more of positive power supply 310 andnegative power supply 312 of system 300.

In the example shown in FIG. 3, each of switching elements 348A-348N maybe controlled by control module 304 to provide the respective writecurrent through the corresponding one of write lines 318A-318N usingwrite control line(s) 306A-306N. Similarly, after providing therespective write current for each of MRAM cells 350A1-350NM, controlmodule 304 may determine a magnetic state of each of MRAM cells350A1-350NM via read line(s) 308A1-308NM.

In some examples, control module 304 is configured to dynamically vary amagnitude and a polarity of the write current provided through each ofwrite lines 318A-318N, and, therefore, a magnitude and a polarity of theresultant write magnetic field induced proximate to the correspondingone of MRAM cells 350A1-350NM, as described in greater detail below withreference to FIG. 8, using write current source 346, sink/source node352, and switching elements 350A1-350NM. In addition, control module 304may be configured to control the relative timing of the write currentsprovided through each of write lines 318A-318N and the resultant writemagnetic fields.

The sensitivity of magnetic field sensing system 300 to externalmagnetic fields may be directly related to the magnitude of the writecurrent provided through each of write lines 318A-318N, the polarity ofthe write current, and the timing of the write current (referred toherein as “sense parameters”). The sensitivity thresholds of magneticfield sensing system 300 may be varied by varying one or more of thesense parameters. For example, magnetic field sensing system 300 may beconfigured to detect an external magnetic field having at least a firstmagnitude when a first set of sense parameters are used, and magneticfield sensing system may be configured to detect an external magneticfield having at least a second magnitude when a second set of senseparameters are used, where the second magnitude is lower than the firstmagnitude. As a result, by varying any one or more of these senseparameters, control module 304, and system 300, generally, may beconfigured to sense magnetic fields having varying magnitudes andpolarities at different times and over varying distances and area withrespect to system 300. This may enable the same system 300 to be readilyadaptable to a variety of applications without expensive and timeconsuming modifications.

In the example of FIG. 3, switching elements 348A-348N may be integratedwithin a common module or unit within MRAM module 302, as depicted bythe dashed lines surrounding switching elements 348A-348N in FIG. 3. Inother examples, switching elements 348A-348N may be separate from MRAMmodule 302 (e.g., in a different enclosure). In addition, in the exampleshown in FIG. 3, MRAM cells 350A1-350NM also may be integrated within acommon module or unit within MRAM module 302, as also depicted by thedashed lines surrounding MRAM cells 350A1-350NM in FIG. 3. In someexamples, although not shown in FIG. 3, MRAM cells 350A1-350NM may beseparated into one or more sub-modules.

An MRAM cell (e.g., any of MRAM cells 350A1-350NM) of system 300 mayhave a magnetic switching threshold, which may, for example, correspondto a minimum magnitude of a magnetic field incident upon the MRAM cellthat is required to change a magnetic state of the MRAM cell. In someexamples, at least two MRAM cells (e.g., any two of 350A1-350NM) mayhave different magnetic switching thresholds. A write line (e.g., any ofwrite lines 318A-318N) associated with the MRAM cell having the magneticswitching threshold may be configured to conduct a write current so asto induce a write magnetic field proximate to the MRAM cell, wherein thewrite magnetic field has a magnitude that is less than the magneticswitching threshold of the MRAM cell. A write current source (e.g., 346)may be electrically coupled to the write line, and a switching element(e.g., any of switching elements 348A-348N) may be electricallyconnected in series with the write line and configured to provide thewrite current from the write current source through the write line.

In other examples, control module 304 may be configured to control oneor more of the write current source and the switching element to providethe write current from the write current source through the write line,such that the first magnitude of the write magnetic field is less thanthe magnetic switching threshold of the MRAM cell. Control module 304may be further configured to, after initiating the provision of thewrite current through the write line, determine a magnetic state of theMRAM cell, and determine a presence of an external magnetic field (e.g.,magnetic field input 314) incident upon the MRAM cell based at least inpart on the determined magnetic state of the MRAM cell. In someexamples, the magnetic state of the MRAM cell may correspond to aresistance of the MRAM cell. In these examples, to determine themagnetic state of the MRAM cell, control module 304 may be configured todetermine the resistance of the MRAM cell.

Additionally, in other examples, the magnetic state of the MRAM cell maycomprise a current magnetic state of the MRAM cell. In these examples,control module 304 may be further configured to, prior to controllingthe one or more of the write current source and the switching element toprovide the write current, determine an initial magnetic state of theMRAM cell. Subsequently, to determine the current magnetic state of theMRAM cell, control module 304 may be configured to determine a change,if any, in the initial magnetic state of the MRAM cell.

In some examples, the magnitude of the write magnetic field may comprisea first magnitude that is proportional to a second magnitude of thewrite current. In these examples, to control the one or more of thewrite current source and the switching element to provide the writecurrent and to determine the magnetic state of the MRAM cell, controlmodule 304 may be configured to control the one or more of the writecurrent source and the switching element to vary iteratively one or moreof the second magnitude of the write current so as to vary the firstmagnitude of the write magnetic field, and a polarity of the writecurrent so as to vary a polarity of the write magnetic field. Controlmodule 304 may be further configured to, for each iteration of varyingthe one or more of the second magnitude and the polarity of the writecurrent, determine a respective magnetic state of the MRAM cell. Inthese examples, to determine the presence of the external magnetic fieldincident upon the MRAM cell, control module 304 may be configured todetermine the presence of the external magnetic field based at least inpart on each of the determined magnetic states of the MRAM cell. Inaddition, to determine one or more properties of the external magneticfield incident upon the MRAM cell, control module 304 may be configuredto determine the one or more properties of the external magnetic fieldbased at least in part on each of the determined magnetic states of theMRAM cell. For example, the determined magnetic states of the MRAM cellin combination with the known magnitude of the write magnetic field mayindicate, for example, the magnitude, polarity, or both, of the externalmagnetic field.

As illustrated by the example of FIG. 3, the MRAM cell may comprise aplurality of MRAM cells with respective magnetic switching thresholds.In some examples, MRAM module 302 includes a first MRAM cell, themagnetic switching threshold may comprise a first magnetic switchingthreshold, the write line may comprise a first write line, the writecurrent may comprise a first write current, the write magnetic field maycomprise a first write magnetic field, the magnitude may comprise afirst magnitude, and the switching element may comprise a firstswitching element. In this example, magnetic field sensing system 300may further comprise a second MRAM cell comprising a second magneticswitching threshold, a second write line associated with the second MRAMcell and configured to conduct a second write current so as to induce asecond write magnetic field proximate to the second MRAM cell, whereinthe second write magnetic field has a second magnitude that is less thanthe second magnetic switching threshold of the second MRAM cell, thewrite current source electrically coupled to the second write line, anda second switching element electrically connected in series with thesecond write line and configured to provide the second write currentfrom the write current source through the second write line.

Additionally, in some examples, the MRAM cell may comprise a magnetictunnel junction (MTJ) MRAM cell, e.g., as described above with referenceto magnetic stack 230 of FIG. 2A. Furthermore, in other examples, thewrite line may comprise one of a bit write line and a digit write line.Finally, in still other examples, the MRAM cell, the write line, and theswitching element may comprise a part of an MRAM device.

In some examples, control module 304 may be further configured to outputone or more signals indicative of one or more properties of the externalmagnetic field, e.g., output signal(s) 316. As one example, outputsignal(s) 316 may include one or more signals indicative of thedetermined presence of the external magnetic field, as described above.As another example, output signal(s) 316 may include one or more signalsindicative of the determined magnitude of the external magnetic field,as also described above. Finally, as still another example, outputsignal(s) 316 may include one or more signals indicative of thedetermined polarity of the external magnetic field, as also describedabove. For example, output signal(s) 316 may be output and/or stored inone or more memories, or memory devices, described above with referenceto system 100 of FIG. 1. For example, output signal(s) 316, whetherrepresented as a single signal, or a plurality of signals, may compriseone or more analog signals, one or more digital signals, or anycombination thereof.

In this manner, magnetic field sensing system 300 of FIG. 3 may beconfigured as a magnetic field sensing system comprising one or moreMRAM cells comprising a respective magnetic switching threshold, a writeline associated with the MRAM cell and configured to conduct a writecurrent so as to induce a write magnetic field proximate to the MRAMcell, wherein the write magnetic field has a magnitude that is less thanthe magnetic switching threshold of the MRAM cell, and a switchingelement electrically connected in series with the write line andconfigured to provide the write current from a write current source.

FIGS. 4-10 are flow diagrams that illustrate example methods of sensinga magnetic field and determining various characteristics thereof using amagnetic field sensing system that includes one or more MRAM cells. Thetechniques of FIGS. 4-10 may be performed by any processing unit orprocessor, whether implemented in hardware, software, firmware, or acombination thereof, and when implemented in software or firmware,corresponding hardware may be provided to execute instructions for thesoftware or firmware. For purposes of example, the techniques of FIGS.4-10 are described with respect to magnetic field sensing systems 100(FIG. 1) and 300 (FIG. 3), MRAM modules 102 (FIG. 1) and 302 (FIG. 3)(e.g., each including one or more MRAM cells 202 of FIG. 2), and controlmodules 104 (FIG. 1) and 304 (FIG. 3), as well as various componentsthereof, although it should be understood that other systems or devicesmay be configured to perform similar techniques. Moreover, the stepsillustrated in FIGS. 4-10 may be performed in a different order or inparallel, and additional steps may be added and certain steps omitted,without departing from the techniques of this disclosure.

FIG. 4 is a flow diagram of a method of sensing an external magneticfield incident upon an MRAM cell, and, in particular, determining apresence of the external magnetic field. As illustrated in FIG. 4, inone example, a control module (e.g., 104, 304) of a magnetic fieldsensing system (e.g., 100, 300) that includes the control module and anMRAM module (e.g., 102, 302) may be configured to sense a magnetic field(e.g., 114, 314) using an MRAM cell (e.g., any of 202, 350A1-350NM). Forexample, the MRAM module of the magnetic field sensing system mayinclude the MRAM cell, as well as one or more additional MRAM cells, aspreviously described with reference to FIGS. 1 and 3.

In the example of FIG. 4, the control module may provide a write currentthrough a write line (e.g., any of 318A-318N) associated with the MRAMcell so as to induce a write magnetic field proximate to the MRAM cell,wherein the write magnetic field has a magnitude that is less than amagnetic switching threshold of the MRAM cell (400).

As previously described with reference to FIGS. 1-3, the magneticswitching threshold of the MRAM cell may correspond to a minimummagnitude of a magnetic field incident upon the MRAM cell that isrequired to change a magnetic state of the MRAM cell. For example, themagnetic switching threshold of the MRAM cell may equal to the minimummagnitude of the magnetic field described above, which may be referredto as a minimum “strength” of the magnetic field. In some examples, theminimum magnitude may be defined as an instantaneous minimum magnitude.In other examples, the minimum magnitude may be defined using otherrepresentations, such as a Root-Mean-Square (RMS), an average, oranother representation. In some examples, the minimum magnitude maycorrespond to a magnitude of an external magnetic field which themagnetic field sensing system is configured to detect, and may be one ofa number of such magnitudes in instances where multiple MRAM cellsincluded within the MRAM module have different magnetic switchingthresholds.

In some examples, the control module provides the write current (400) byat least controlling one or more of a write current source electricallycoupled to the write line (e.g., write current source 346) and aswitching element electrically connected in series with the write line(e.g., any of 348A-348N) and configured to provide the write currentfrom the write current source through the write line, to provide thewrite current through the write line. As one example, the control modulemay control the write current source to generate the write current suchthat the magnitude of the write magnetic field is less than the magneticswitching threshold of the MRAM cell, and control the switching elementto deliver the generated write current. As another example, the writecurrent source may provide a supply current, and the control module maycontrol the switching element to modify the supply current to generatethe write current, such that the magnitude of the write magnetic fieldis less than the magnetic switching threshold of the MRAM cell. In otherwords, in some examples, the control module may control the switchingelement so as to modify (e.g., modulate, or vary an amplitude of, or“step”) another current (e.g., a supply current having a fixedamplitude) to generate the write current.

As described above with reference to FIG. 3, the write current source,as well as a sink/source node (e.g., sink/source node 352) operable inconjunction with the write current source, may correspond to, or bederived from, one or more of a positive power supply (e.g., positivepower supply 110 or 310) and a negative power supply (e.g., negativepower supply 112 or 312) also included within the magnetic field sensingsystem. Furthermore, the switching element may include one or more of aBJT-based switching device, a MOSFET-based switching device, or otherdevice or circuitry for providing the switching functionality describedabove with reference to the switching element. As also described above,in some examples, the switching element may itself comprise a currentsource configured to provide the write current through the write line,e.g., by varying a supply current from the write current source, or fromanother source, to generate the write current.

After initiating the provision of the write current through the writeline, the control module may further determine a magnetic state of theMRAM cell (402). In some examples, the control module may determine themagnetic state of the MRAM cell during the provision of the writecurrent, e.g., while the one or more of the write current source and theswitching element are providing the write current through the writeline. In other examples, the control module may determine the magneticstate of the MRAM cell after the provision of the write current, e.g.,following the completion of the provision of the write current throughthe write line by the one or more of the write current source and theswitching element. In this manner, some techniques of this disclosureinclude determining the magnetic state of the MRAM cell while providingthe write current through the write line associated with the MRAM cell(e.g., during a write cycle of the MRAM cell), as well as at asubsequent point in time (e.g., during a read cycle of the MRAM cell, asdescribed above with reference to FIG. 2). Accordingly, the techniquesdescribed herein may include providing the write current through thewrite line associated with the MRAM cell and determining the magneticstate of the MRAM cell both contemporaneously and at different times.

In some examples, as described above with reference to FIG. 2, themagnetic state of the MRAM cell may correspond to a resistance of theMRAM cell. In these examples, to determine the magnetic state of theMRAM cell, the control module may determine the resistance of the MRAMcell, e.g., using the techniques for determining the resistance ofmagnetic stack 230 described above with reference to FIG. 2A.

As shown in FIG. 4, in response to determining the magnetic state of theMRAM cell has changed (404) (“YES”), the control module may determine apresence of an external magnetic field (e.g., 114, 314) incident uponthe MRAM cell based at least in part on the determined magnetic state ofthe MRAM cell (406). On the other hand, in response to determining themagnetic state of the MRAM cell has not changed (404) (“NO”), thecontrol module may determine that no external magnetic field has beendetected (408).

In some examples, the determined magnetic state of the MRAM cell (402)may be referred to as a current magnetic state of the MRAM cell, e.g.,the magnetic state of the MRAM cell at the time the control moduledetermines the magnetic state. In these examples, the control module mayfurther, prior to providing the write current (400), determine aninitial magnetic state of the MRAM cell. In these examples, to determinethe current magnetic state of the MRAM cell, the control module maydetermine a change in the initial magnetic state of the MRAM cell.

For example, in response to determining that the current magnetic stateof the MRAM cell is the same as the initial magnetic state, the controlmodule may determine that no external magnetic field has been detected(408). For example, the control module may determine that the MRAM cellwas not exposed to an external magnetic field that has a magnitude whichthe magnetic field sensing system, or at least a subset of the systemincluding the particular MRAM cell whose magnetic state did not change,is configured to detect. On the other hand, in response to determiningthat the current magnetic state of the MRAM cell is different than theinitial magnetic state, the control module may determine a presence ofthe external magnetic field incident upon the MRAM cell (406). Thecontrol module may determine that the MRAM cell was exposed to anexternal magnetic field that has, at a minimum, the magnitude which themagnetic field sensing system is configured to detect.

In the example of FIG. 4, in response to determining the presence of theexternal magnetic field incident upon the MRAM cell (406), the controlmodule may output one or more output signal(s) indicative of thedetected presence of the external magnetic field (410).

FIG. 5 is a flow diagram of a method of sensing an external magneticfield incident upon an MRAM cell, and, in particular, determining apresence and a characteristic of the external magnetic field. As withthe technique described with respect to FIG. 4, in the method shown inFIG. 5, the control module provides a write current through a write lineassociated with an MRAM cell (400), determines a magnetic state of theMRAM cell (402), determines whether the magnetic state of the MRAM cellhas changed (404), and determines one of a presence of an externalmagnetic field incident upon the MRAM cell (406), and that no externalmagnetic field has been detected (408) based on the magnetic state ofthe MRAM cell.

In the method of FIG. 5, if the control module determines the presenceof an external magnetic field (406), the control module determines acharacteristic of the external magnetic field based on a characteristicof the write current and a characteristic of the MRAM cell (412). Forexample, as described above, the characteristic of the write current mayinclude one or more of a magnitude and a polarity of the write current,and the characteristic of the MRAM cell may include a magnetic switchingthreshold of the MRAM cell. Additionally, as also described above, andas will be described in greater detail below with reference to FIGS. 6and 7, the characteristic of the external magnetic field may include oneor more of a magnitude and a polarity of the external magnetic field. Inthe example of FIG. 5, the control module may further output one or moreoutput signal(s) indicative of the determined characteristic of theexternal magnetic field (414).

FIG. 6 is a flow diagram of an example of the method of FIG. 5 andillustrates an example technique for determining a characteristic of theexternal magnetic field based on a characteristic of the write currentand a characteristic of the MRAM cell (block 412 in FIG. 5). As shown inFIG. 6, the method includes providing a write current through a writeline associated with an MRAM cell (400), determining a magnetic state ofthe MRAM cell (402), determining whether the magnetic state of the MRAMcell has changed (404), and determining one of a presence of an externalmagnetic field incident upon the MRAM cell (406) and that no externalmagnetic field has been detected (408).

In the method of FIG. 6, to determine the characteristic of the externalmagnetic field based on the characteristic of the write current and thecharacteristic of the MRAM cell (block 412 in FIG. 5), the controlmodule determines a magnitude of the external magnetic field. In otherwords, in the example of FIG. 6, the characteristic of the externalmagnetic field determined by the control module comprises a magnitude ofthe external magnetic field. As described above, the characteristic ofthe write current may include one or more of a magnitude and a polarityof the write current, and the characteristic of the MRAM cell mayinclude a magnetic switching threshold of the MRAM cell.

In particular, in the example of FIG. 6, in the event the presence ofthe external magnetic field incident upon the MRAM cell is detected(406), to determine the characteristic of the external magnetic fieldbased on the characteristic of the write current and the characteristicof the MRAM cell (block 412 in FIG. 5), the control module may determinea difference between the magnetic switching threshold of the MRAM celland the magnitude of the write magnetic field (416), which, as discussedabove, is proportional to the magnitude of the write current. Thecontrol module may further determine the magnitude of the externalmagnetic field based at least in part on the determined difference(418).

As discussed above, the magnitude of the write current is proportionalto the magnitude of the write magnetic field. In some examples,determining the presence of the external magnetic field as describedabove may indicate that the sum of the magnitudes of the externalmagnetic field and the write magnetic field is greater than or equal tothe magnetic switching threshold of the MRAM cell. As such, upondetermining the presence of the external magnetic field (406), thecontrol module (e.g., control module 104 or 304) may determine themagnitude of the external magnetic field based on a difference betweenthe magnitude of the write magnetic field and the magnetic switchingthreshold of the MRAM cell. In some examples, to determine the magnitudeof the external magnetic field, the control module may utilize thefollowing expression:

Ext_Field_(MAG)>=[Field_(TH)]−[Write_Field_(MAG)]  (Equation 1)

Where Ext_Field_(MAG) is the magnitude of the external magnetic field,Field_(TH) is the magnetic switching threshold of the MRAM cell,Write_Field_(MAG) is the magnitude of the write magnetic field, and the“>=” symbol indicates that Ext_Field_(MAG) is greater than or equal tothe difference “[Field_(TH)]−[Write_Field_(MAG)].” Accordingly, in someexamples, the magnitude of the external magnetic field may be expressedas a magnitude of a magnetic field that, in conjunction with themagnitude of the write magnetic field, is required to reach or exceed(i.e., be greater than) the magnetic switching threshold of the MRAMcell, and thereby cause the MRAM cell to change its magnetic state.

As illustrated by the “>=” symbol in Equation 1 above, in some examples,the magnitude of the external magnetic field, in conjunction with themagnitude of the write magnetic field, may not merely reach, but also begreater than the magnetic switching threshold of the MRAM cell, andthereby cause the MRAM cell to change its magnetic state, as describedabove. In these examples, to determine the magnitude of the externalmagnetic field based on the difference between the magnetic switchingthreshold of the MRAM cell and the magnitude of the write magneticfield, the control module may require iteratively varying the magnitudeof the write current (e.g., iteratively increasing the magnitude of thewrite current from a minimum value to a maximum value), until the MRAMcell changes its magnetic state, as described in greater detail withreference to FIG. 8. In these examples, the accuracy of the determinedmagnitude of the external magnetic field may depend on, e.g., an amount,or “step-size,” by which the magnitude of the write current is variedfor each iteration. For example, smaller step-sizes may result ingreater accuracy of the determined magnitude of the external magneticfield. In other examples, the accuracy may also depend on a frequency ofthe iterations, as well as a frequency of the correspondingdeterminations of the magnetic state of the MRAM cell, which may bereferred to as a sampling rate of the magnetic state. This may be thecase, for example, in instances where the magnitude of external magneticfield varies over time with respect to the MRAM cell. In this manner,the determined magnitude of the external magnetic field may be referredto as an “estimated,” rather than an actual magnitude of the externalmagnetic field.

In the example shown in FIG. 6, the control module may output one ormore output signal(s) indicative of the determined magnitude of theexternal magnetic field (420). In this manner, the example method ofFIG. 6 illustrates an example of a method of determining a magnitude ofthe external magnetic field based at least in part on the magnitude ofthe write current and the magnetic switching threshold of the MRAM cell.

FIG. 7 is a flow diagram of another example of the method of FIG. 5 andillustrates an example technique for determining a presence and apolarity of the external magnetic field. The method of FIG. 7 includesthe previously described steps of the method of FIG. 5 that relate tothe control module providing a write current through a write lineassociated with an MRAM cell (400), determining a magnetic state of theMRAM cell (402), determining whether the magnetic state of the MRAM cellhas changed (404), and determining one of a presence of an externalmagnetic field incident upon the MRAM cell (406) and that no externalmagnetic field has been detected (408).

In the method of FIG. 7, to determine the characteristic of the externalmagnetic field based on the characteristic of the write current and thecharacteristic of the MRAM cell (block 412 in FIG. 5), the controlmodule determines a polarity of the external magnetic field. In otherwords, in the example of FIG. 7, the characteristic of the externalmagnetic field comprises a polarity of the external magnetic field. Onceagain, as described above, the characteristic of the write current mayinclude one or more of a magnitude and a polarity of the write current,and the characteristic of the MRAM cell may include a magnetic switchingthreshold of the MRAM cell.

As shown in FIG. 7, in the event the presence of the external magneticfield incident upon the MRAM cell is determined (406), to determine thecharacteristic of the external magnetic field based on thecharacteristic of the write current and the characteristic of the MRAMcell (block 412 shown in FIG. 5), the control module (e.g., controlmodule 104 or 304) may determine a polarity of one or more of the writecurrent and the write magnetic field (422). The control module mayfurther determine the polarity of the external magnetic field based atleast in part on the determined polarity of the one or more of the writecurrent and the write magnetic field (424).

As explained above, the magnitude of the write current is proportionalto the magnitude of the write magnetic field. Moreover, the polarity ofthe write current is also determinative of, i.e., is the same as, thepolarity of the write magnetic field. For example, the polarity of thewrite magnetic field may be determined based on a direction, and,therefore, the polarity of the write current using the conventional“right-hand rule.” In some examples, determining the presence of theexternal magnetic field as described above may indicate that theexternal magnetic field and the write magnetic field have a commonpolarity, such that the sum of the magnitudes of the respective fieldsis greater than or equal to the magnetic switching threshold of the MRAMcell. As such, upon determining the presence of the external magneticfield (406), the control module (e.g., control module 104 or 304) mayfurther determine the polarity of the external magnetic field based onthe polarity of the write current and/or the polarity of the writemagnetic field, which, as discussed above, are determinative of oneanother. For example, to determine the polarity of the external magneticfield, the control module may utilize one of the following expressions:

Ext_Field_(POL)=SIGN[Write_Field_(MAG)]  (Equation 2A)

Ext_Field_(POL)=SIGN[Write_Current_(MAG)]  (Equation 2B)

Where Ext_Field_(POL) is the polarity of the external magnetic field,Write_Field_(MAG) is the magnitude of the write magnetic field,Write_Current_(MAG) is the magnitude of the write current, and SIGNindicates a sign operator used to determine a sign of each ofWrite_Field_(MAG) and Write_Current_(MAG). In some examples, thepolarity of the external magnetic field may be represented using a sign(e.g., “+” or “−”) that corresponds to the polarity of the externalmagnetic field. In other examples, the polarity of the external magneticfield may be expressed as one or more vectors indicating the polarity ofthe external magnetic field.

In the example shown in FIG. 7, the control module may output one ormore output signal(s) indicative of the determined polarity of theexternal magnetic field (426). In this manner, the example method ofFIG. 7 illustrates an example of a method of determining a polarity ofthe external magnetic field based at least in part on a polarity of thewrite current.

FIG. 8 is a flow diagram of a method of sensing an external magneticfield incident upon an MRAM cell, and, in particular, determining apresence and a characteristic of the external magnetic field byiteratively varying the write current. Specifically, in the example ofFIG. 8, the magnitude of the write magnetic field may comprise a firstmagnitude that is proportional to a second magnitude of the writecurrent. In this example, to provide the write current and determine themagnetic state of the MRAM cell, the control module may iteratively varyone or more of the second magnitude of the write current so as to varythe first magnitude of the write magnetic field, and a polarity of thewrite current so as to vary a polarity of the write magnetic field. Inthis example, for each iteration of varying the second magnitude, thepolarity of the write current, or both, the control module may determinea respective magnetic state of the MRAM cell. In the method shown inFIG. 8, to determine the presence of the external magnetic fieldincident upon the MRAM cell, the control module may determine thepresence of the external magnetic field based at least in part on thedetermined magnetic states of the MRAM cell. In some examples, thecontrol module may further determine one or more characteristics of theexternal magnetic field, which may include one or more of a thirdmagnitude and a polarity of the external magnetic field, e.g., asdescribed above with reference to FIGS. 6 and 7.

In the technique shown in FIG. 8, for each magnitude, polarity, orcombination of magnitude and polarity, selected by the control module,the control module provides a write current through a write lineassociated with an MRAM cell (not shown in FIG. 8), determines amagnetic state of the MRAM cell (402), determines whether the magneticstate of the MRAM cell has changed (404), determines a presence of anexternal magnetic field incident upon the MRAM cell (406) or that noexternal magnetic field has been detected (408), and, in response todetermining the presence of an external magnetic field, determines acharacteristic of the external magnetic field based on a characteristicof the write current and a characteristic of the MRAM cell (412). In theexample of FIG. 8, to provide the write current through the write lineassociated with the MRAM cell (400), the control module varies one ofmore of a magnitude and a polarity of the write current (428).

In response to determining the magnetic state of the MRAM cell has notchanged (“NO” branch of block 404), the control module may repeat (i.e.,iterate) the above-described steps (428), (402), and (404) until thecontrol module has completed (i.e., “is done”) iteratively varying theone of more of the magnitude and the polarity of the write current(“YES” branch of block 430). For example, in response to determining themagnetic state of the MRAM cell has not changed (404) (“NO”), thecontrol module may vary a magnitude of the write current, a polarity ofthe write current, or both the magnitude and polarity of the writecurrent and provide the write current to the write line for the MRAMcell (428), and, subsequently, determine a magnetic state of the MRAMcell (402), and determine whether the magnetic state has changed (404) apredetermined number of times, after which point the control module maydetermine that no external magnetic field has been detected (408). Thepredetermined number of times may be selected to be suitable for themagnetic field sensing system.

In some examples, as shown in FIG. 8, in response to determining apresence of the external magnetic field incident upon the MRAM cell(406) (i.e., in the event the magnetic state of the MRAM cell haschanged (“YES” branch of block 404)), the control module may determine acharacteristic of the external magnetic field based on a characteristicof the write current (selected by control module at block 428 shown inFIG. 8) that caused the change of the magnetic state of the MRAM cell,as well as the characteristic of the MRAM cell (432).

As one example, the control module may iteratively increase themagnitude of the write current from a minimum value to a maximum value(428) so as to correspondingly increase the magnitude of the inducedwrite magnetic field, until the magnetic state of the MRAM cell changesto a different magnetic state. As another example, the control modulemay iteratively vary the polarity of the write current (428) so as tovary the polarity of the induced write magnetic field until the magneticstate of the MRAM cell changes to a different magnetic state. As yetanother example, the control module may perform a combination of theabove-described iterative variations of the magnitude and the polarityof the write current (e.g., perform a magnitude “sweep” for eachpolarity by switching the polarity of the write current for each writecurrent magnitude) (428) until the magnetic state of the MRAM cellchanges to a different magnetic state.

In this manner, by varying the one or more of the magnitude and thepolarity of the write current, the magnetic field sensing system may beconfigured to not only detect a presence of an external magnetic fieldhaving a magnitude greater than or equal to the magnetic switchingthreshold of the MRAM cell, but also determine one or more of amagnitude and a polarity of the external magnetic field, as discussedabove with reference to FIGS. 6 and 7. In each instance, the controlmodule may perform the particular iterative variation(s) described aboveuntil the magnetic state of the MRAM cell changes to a differentmagnetic state, or until no change in the magnetic state of the MRAMcell is detected, in which case the control module may determine that nomagnetic field has been detected (408).

In the example shown in FIG. 8, the control module may output one ormore output signal(s) indicative of the determined characteristic of theexternal magnetic field (414).

In some examples, such as those described in greater detail below withreference to FIGS. 9 and 10, the MRAM module may include a plurality ofMRAM cells. In these examples, the MRAM cell described above withreference to FIGS. 4-8 may comprise a first MRAM cell. Similarly, themagnetic switching threshold may comprise a first magnetic switchingthreshold, the write line may comprise a first write line, the writecurrent may comprise a first write current, the write magnetic field maycomprise a first write magnetic field, and the magnitude may comprise afirst magnitude. In these examples, the control module may furtherprovide a second write current through a second write line associatedwith a second MRAM cell so as to induce a second write magnetic fieldproximate to the second MRAM cell, wherein the second write magneticfield has a second magnitude that is less than a second magneticswitching threshold of the second MRAM cell. The control module may,after initiating the provision of the second write current through thesecond write line, determine a magnetic state of the second MRAM cell.The control module may determine a presence of an external magneticfield incident upon the second MRAM cell based at least in part on thedetermined magnetic state of the second MRAM cell.

In some examples, the provision of the first and second write currentsdescribed above may be performed simultaneously or at different times.Additionally, in some examples, the external magnetic field incidentupon each of the first and second MRAM cells may be a same externalmagnetic field. In other examples, the external magnetic field incidentupon the first MRAM cell may be different than the external magneticfield incident upon the second MRAM cell.

In some examples, the first magnetic switching threshold may bedifferent than the second magnetic switching threshold, the first writecurrent may be different than the second write current, or both thefirst magnetic switching threshold may be different than the secondmagnetic switching threshold and the first write current may bedifferent than the second write current. Accordingly, in some examplesin which the MRAM module includes a plurality of MRAM cells, two or moreMRAM cells of the plurality of MRAM cells may be configured to havedifferent magnetic switching thresholds, two or more of the writecurrents provided through the write lines associated with the pluralityof MRAM cells may be different (i.e., have different magnitudes and/orpolarities), or any combination thereof. As a result, the two or moreMRAM cells may be configured to detect magnetic fields having differentproperties (e.g., different absolute magnitudes, polarities, or both).

FIGS. 9 and 10 are flow diagrams of example methods of sensing externalmagnetic fields and various properties thereof in examples in which theMRAM module includes a plurality of MRAM cells. In the example of FIG.9, each MRAM cell of the plurality of MRAM cells has a differentmagnetic switching threshold, but is associated with a common writeline. In the example of FIG. 10, each MRAM cell of the plurality of MRAMcells has a same, or a substantially same magnetic switching threshold,and is associated with a common write line.

Other examples not illustrated in FIGS. 9 and 10 include any combinationof the plurality of MRAM cells having same or different magneticswitching thresholds and being associated with same or different writelines. Moreover, in the example of each of FIGS. 9 and 10, the pluralityof MRAM cells may be subset of a plurality of MRAM cells included withinthe MRAM module, which may include additional MRAM cells (e.g., MRAMcells that have same or different magnetic switching thresholds and thatare associated with same or different write lines, including MRAM cellsused for data storage).

As described below with reference to FIG. 9, the control module may, insome examples, determine one or more characteristics of the externalmagnetic field, which may include one or more of a magnitude and apolarity of the external magnetic field, e.g., using the techniquesdescribed above with reference to FIGS. 6 and 7.

FIG. 9 is a flow diagram illustrating an example of a method of sensingan external magnetic field incident upon a plurality (e.g., an array) ofMRAM cells, and, in particular, determining a presence and acharacteristic of the external magnetic field. The method of FIG. 9includes steps that are similar to the previously described steps of themethod of FIG. 4 that relate to the control module providing a writecurrent through a write line associated with an MRAM cell (400),determining a magnetic state of the MRAM cell (402), determining whetherthe magnetic state of the MRAM cell has changed (404), and determiningone of a presence of an external magnetic field incident upon the MRAMcell (406), and that no external magnetic field has been detected (408).However, in the example of FIG. 9, the control module provides a writecurrent through a write line associated with a plurality of MRAM cells(434), determines a magnetic state of each MRAM cell of the plurality ofMRAM cells (436), and determines whether the magnetic state of any ofthe plurality of MRAM cells has changed (438). In the technique shown inFIG. 9, the control module determines a presence of an external magneticfield incident upon the plurality of MRAM cells (440) or that noexternal magnetic field has been detected (408) based on the magneticstate of each MRAM cell of the plurality of MRAM cells. The techniqueshown in FIG. 9 may be implemented with an MRAM module (e.g., MRAMmodule 302 shown in FIG. 3) that includes a plurality of MRAM cells,where the write line is associated with (i.e., is common to) each MRAMcell of the plurality of MRAM cells. In some examples, two or more ofthe MRAM cells of the plurality of MRAM cells may have differentmagnetic switching thresholds, as described in greater detail below.

In some examples, as shown in FIG. 9, in response to detecting thepresence of the external magnetic field incident upon the plurality ofMRAM cells (440), the control module determines a characteristic of theexternal magnetic field (e.g., the magnitude, polarity, or both) basedon a characteristic of the write current and a characteristic of atleast one MRAM cell of the plurality of MRAM cells (442). Thecharacteristic of the write current may include one or more of amagnitude and a polarity of the write current, and the characteristic ofeach MRAM cell of the plurality of MRAM cells may include a magneticswitching threshold of the respective MRAM cell.

In the example of FIG. 9, two or more MRAM cells of the plurality ofMRAM cells magnetically coupled to a common write line may havedifferent magnetic switching thresholds. In contrast to the techniqueshown in FIG. 8, by including at least two MRAM cells with differentmagnetic switching thresholds, a magnetic field sensing system may beconfigured to determine the presence and the one or more of themagnitude and the polarity of an external magnetic field via thedelivery of one write current. For example, a first MRAM cell of theplurality of MRAM cells that has a first magnetic switching thresholdmay change its magnetic state, while a second MRAM cell of the pluralityof MRAM cells that has a second, different magnetic switching thresholdmay not change its magnetic state. In this example, the presence of theexternal magnetic field may be determined based on the change of themagnetic state of the first MRAM cell. Additionally, in this example,the magnitude of the external magnetic field may be determined based onthe first MRAM cell changing its magnetic state, and the second MRAMcell not changing its magnetic state. In examples in which the firstmagnetic switching threshold is less than the second magnetic switchingthreshold, the control module may determine a range of the magnitude ofthe external magnetic field based on the first and second magneticswitching thresholds and the magnitude of the write current. Forexample, the control module may determine the magnitude of the externalmagnetic field to be greater than or equal to the difference between themagnitude of the write magnetic field induced by the write current andthe first magnetic switching threshold, and less than the differencebetween the magnitude of the write magnetic field induced by the writecurrent and the second magnetic switching threshold.

In the example of FIG. 9, the control module may determine thecharacteristic (i.e., the magnitude and/or polarity) of the externalmagnetic field based on the characteristic (i.e., the magnitude and/orpolarity) of the write current and the characteristic (i.e., themagnetic switching threshold) of the at least one of the plurality ofMRAM cells (442) that either changed or did not change its magneticstate, e.g., at least one MRAM cell that changed its magnetic state, andat least one MRAM cell that did not change its magnetic state.

In other examples, the MRAM module may include additional MRAM cellshaving magnetic switching thresholds that are different than the firstand second magnetic switching thresholds. In these examples, themagnitude of the external magnetic field may be determined to be greaterthan or equal to a difference between the magnitude of the writemagnetic field induced by the write current and the magnetic switchingthreshold of a particular MRAM cell that changed state in response tothe delivery of the write current, and less than a difference betweenthe magnitude of the write magnetic field induced by the write currentand the magnetic switching threshold of another MRAM cell that did notchange state in response to the delivery of the write current. In thismanner, in examples in which the MRAM module includes a large number ofMRAM cells each comprising a different magnetic switching threshold, themagnetic field sensing system may be configured to sense a range ofmagnetic field magnitudes in fine increments, and the sensing system maysense and determine a magnitude of the external magnetic field withinthis range with a relatively high degree of accuracy compared toexamples in which a single MRAM cell is used to determine the magnitudeof the external magnetic field.

Moreover, the above-described techniques may be performed for variousmagnitudes and polarities of the write current, such that a magneticfield sensing system may be configured to detect not only a range ofexternal magnetic field magnitudes, but also external magnetic fieldshaving different polarities. In this manner, the control module maydetermine the characteristic of the external magnetic field based on thecharacteristic (i.e., the particular magnitude and/or polarity) of thewrite current, and the characteristic of the at least one of theplurality of MRAM cells (442), as described above. In some examples, asshown in FIG. 9, the control module may output one or more outputsignal(s) indicative of the determined characteristic of the externalmagnetic field (414).

FIG. 10 is a flow diagram of another example method of sensing anexternal magnetic field incident upon a plurality of MRAM cells, and, inparticular, determining a presence and an approximate location of theexternal magnetic field relative to the plurality of MRAM cells. Asshown in FIG. 10, the method of FIG. 10 includes the previouslydescribed steps of the method of FIG. 9 that relate to the controlmodule providing a write current through a write line associated with aplurality of MRAM cells (434), determining a magnetic state of each ofthe plurality of MRAM cells (436), determining whether the magneticstate of any of the plurality of MRAM cells has changed (438), anddetermining one of a presence of an external magnetic field incidentupon the plurality of MRAM cells (440), and that no external magneticfield has been detected (408). Once again, in the example of FIG. 10,the MRAM module includes a plurality of MRAM cells, and the write lineis associated with each of the plurality of MRAM cells. In this example,however, each of the plurality of MRAM cells has a same, or asubstantially same magnetic switching threshold, as will be described ingreater detail below.

The method of FIG. 10 includes, in the event the presence of theexternal magnetic field incident upon the plurality of MRAM cells isdetermined (440), the control module determining a location of each ofthe plurality of MRAM cells for which the magnetic state has changed(444), and determining an approximate location of the external magneticfield based on the determined location (446). For example, the controlmodule may determine a location of one or more of the plurality of MRAMcells for which the magnetic state has changed, indicating that the oneor more of the plurality of MRAM cells have been exposed to the externalmagnetic field.

As discussed above, in the example of FIG. 10, each of the plurality ofMRAM cells has a same, or a substantially same magnetic switchingthreshold. In this example, the determined location of each of theplurality of MRAM cells for which the magnetic state has changed may beindicative of the presence of the external magnetic field at therespective location. In this manner, once again, by including theplurality of MRAM cells within the MRAM module, but, in this case, bydesignating a same, or a substantially same magnetic switching thresholdfor each of the plurality of MRAM cells, the magnetic field sensingsystem may be configured to determine the presence and the approximatelocation of the external magnetic field relative to the plurality ofMRAM cells. For example, a particular one of the one or more of theplurality of MRAM cells may be located at a point along the write line,which may be indicative of the approximate location of the externalmagnetic field relative to respective MRAM cell. Similarly, the one ormore of the plurality of MRAM cells may be concentrated, or otherwisegrouped in a region along the write line, e.g., the one or more of theplurality of MRAM cells may include two or more MRAM cells that areadjacent along the write line. Such a grouping may be indicative of theapproximate location (and, e.g., a range or a “span” over distance) ofthe external magnetic field relative to the plurality of MRAM cells, andto the MRAM module generally.

Additionally, in the above-described examples, the plurality of MRAMcells may be arranged within the MRAM module along the write line so asto cover a particular range, or a distance, within the MRAM module,wherein each of the plurality of MRAM cells is placed along the writeline in fine increments of distance. As a result, the approximatelocation of the external magnetic field within this range or distancerelative to the plurality of MRAM cells may be determined with a highdegree of accuracy.

Once again, the above-described techniques may be performed for variousmagnitudes and polarities of the write current. In this manner, thecontrol module may determine the presence of the external magnetic fieldincident upon the plurality of MRAM cells (440) for the particularmagnitude and/or polarity of the write current, and proceed to determinethe location of each of the plurality of MRAM cells for which themagnetic state has changed (444), and determine the approximate locationof the external magnetic field based on the determined location (446),as described above. Finally, the control module may further output oneor more output signal(s) indicative of the determined location of theexternal magnetic field (448).

In other examples consistent with the techniques of this disclosure, themagnetic field sensing system including the MRAM module comprising theplurality of MRAM cells, as illustrated in the examples of each of FIGS.9 and 10, may be configured to determine presence, magnitude, andpolarity of multiple external magnetic fields (e.g., multiple externalmagnetic fields of varying magnitudes and/or polarities) incident uponthe plurality of MRAM cells.

As explained above, in the examples of FIGS. 4-10, the control modulemay output one or more output signal(s) (e.g., control module outputsignal(s) 116) indicative of the determined presence, characteristic(i.e., the magnitude and/or the polarity), and location of the externalmagnetic field. In these examples, the control module may output the oneor more output signal(s) in response to an interrogation signal, or thecontrol module may initiate the output of the one or more outputsignal(s). Furthermore, the control module may output the one or moreoutput signal(s) described above, and/or store the one or more outputsignal(s) in one or more memories, or memory devices (e.g., one or moreMRAM cells of MRAM module 202 used for data storage, as opposed tomagnetic field sensing), described above with reference to system 100 ofFIG. 1. For example, the one or more output signal(s), whetherrepresented as a single signal, or a plurality of signals, may compriseone or more analog signals, one or more digital signals, or anycombination thereof.

In this manner, the method of each of FIG. 4-10 represents an example ofa method of sensing a magnetic field, the method comprising providing awrite current through a write line associated with an MRAM cell so as toinduce a write magnetic field proximate to the MRAM cell, wherein thewrite magnetic field has a magnitude that is less than a magneticswitching threshold of the MRAM cell, determining a magnetic state ofthe MRAM cell after initiating the provision of the write currentthrough the write line, and determining a presence of an externalmagnetic field incident upon the MRAM cell based at least in part on themagnetic state of the MRAM cell.

The techniques of the example methods of FIGS. 4-10 may be used todetect presence, magnitude, and polarity of magnetic fields, such asexternal magnetic fields, which may be generated by sources external tothe magnetic field sensing system. For example, during shipment of adevice, system, container, or the like, including system 100 or 300,control module 104 or 304 may periodically provide one or more writecontrol signals to the respective one of MRAM modules 102, 302 to senseone or more magnetic fields and determine various properties thereof,using one or more read signals. This may enable system 100 or 300 todetect tampering with the device, system, or container in which system100 or 300 is included, or otherwise determine if the device, system, orcontainer has been exposed to magnetic fields having magnitudes greaterthan or equal to a threshold magnitude which the system 100 or 200 isconfigured to sense (e.g., based on properties of write current(s) andmagnetic switching threshold(s) of one or more MRAM cells includedwithin the respective one of MRAM modules 102, 302). Detecting exposureof a device, system, or container to a particular external magneticfield may help determine whether the integrity of the device, system, orproducts within the container may have been compromised, e.g., due totampering or due to changes to the device, system, or products withinthe container, that may incidentally occur due to exposure to externalmagnetic fields.

Example applications of the magnetic field sensing techniques of thisdisclosure include, but are not limited to, detecting and bufferingexternal magnetic field disturbances to a device or system (e.g.,disturbances from stray magnetic fields, or from purposefully appliedmagnetic fields), and detecting and buffering exposures of the device orsystem to external magnetic fields. As one example, during transport ofa device, the device may be exposed to stray magnetic fields. In thisexample, the exposure of the device to the stray magnetic fields maycorrupt, or otherwise compromise data stored within the device, orwithin one or more components of the device. As another example, adevice may be interrogated using purposefully applied magnetic fields,e.g., so as to characterize the device in some manner, or to damage thedevice. In this example, the characterization or damaging of the devicemay expose proprietary data stored within the device, or destroy thedevice itself.

The techniques of this disclosure may prevent or reduce the likelihoodof the above-described adverse effects of exposure to magnetic fields byenabling the magnetic field sensing system including the control moduleand the MRAM module, as described above, to sense magnetic fields, and,in particular, to determine one or more of presence, magnitude, andpolarity of magnetic fields, using an MRAM cell (e.g., one or more MRAMcells included within the MRAM module). For example, the techniquesdescribed herein may enable notifying a user of a system or a devicethat includes the magnetic field sensing system of this disclosure thatthe system or device has been exposed to an external magnetic field, aswell as indicating one or more of a magnitude and a polarity of theexternal magnetic field, to the user. In contrast to other magneticfield sensing devices, systems, and techniques, e.g., those using MR orHall-Effect technologies previously described, the devices, systems, andtechniques of this disclosure may enable sensing magnetic fields, and,more specifically, determining any of the above-described properties ofmagnetic fields, using one or more MRAM cells. Additionally, asdescribed above, the techniques of this disclosure may be used as partof systems and methods for protecting the integrity of data storedwithin devices, including MRAM devices, as well as to provide devicetamper protection of the devices.

The techniques of this disclosure may be implemented in a wide varietyof computer devices. Any components, units, or modules that have beendescribed are provided to emphasize functional aspects, and do notnecessarily require realization by different components, units, ormodules. The techniques described herein may also be implemented inhardware, software, firmware, or any combination thereof. Any featuresdescribed as modules, units, or components may be implemented togetherin an integrated logic device, or separately as discrete butinteroperable logic devices. In some cases, various features may beimplemented as an integrated circuit (IC) device, such as an IC chip, orchipset.

If any aspects of the techniques are implemented in software, thetechniques may be realized at least in part by a computer-readablestorage medium comprising instructions that, when executed in aprocessor, perform one or more of the methods described above. Thecomputer-readable storage medium may comprise a tangible, ornon-transitory, computer-readable storage medium, and may form part of alarger product. The computer-readable storage medium may comprise randomaccess memory (RAM) such as synchronous dynamic random access memory(SDRAM), read-only memory (ROM), non-volatile random access memory(NVRAM), electrically erasable programmable read-only memory (EEPROM),FLASH memory, magnetic (e.g., MRAM) or optical data storage media, andthe like. The computer-readable storage medium may also comprise anon-volatile storage device, such as a hard-disk, magnetic tape, acompact disc (CD), a digital versatile disc (DVD), a Blu-ray disc,holographic data storage media, or other non-volatile storage device.

The memory, or memory devices, described herein, which may be used aspart of the described techniques, may also be realized in any of a widevariety of memory, or memory devices, including but not limited to, RAM,SDRAM, NVRAM, EEPROM, FLASH memory, dynamic RAM (DRAM), magnetic RAM(MRAM), or other types of memory.

The term “processor” as used herein may refer to any of the foregoingstructure or any other structure suitable for implementation of thetechniques described herein. In addition, in some aspects, thefunctionality described herein may be provided within dedicated softwaremodules or hardware modules configured for performing the techniques ofthis disclosure. Even if implemented in software, the techniques may usehardware such as a processor to execute the software, and a memory, ormemory device, to store the software. In any such cases, the computersdescribed herein may define a specific machine that is capable ofexecuting the specific functions described herein. Also, the techniquescould be fully implemented in one or more circuits or logic elements,which could also be considered a processor.

Various examples have been described. These and other examples arewithin the scope of the following claims.

What is claimed is:
 1. A magnetic field sensing system comprising: amagnetoresistive random access memory (MRAM) cell comprising a magneticswitching threshold; a write line associated with the MRAM cell andconfigured to conduct a write current so as to induce a write magneticfield proximate to the MRAM cell, wherein the write magnetic field has amagnitude that is less than the magnetic switching threshold of the MRAMcell; and a switching element electrically connected in series with thewrite line and configured to provide the write current from a writecurrent source.
 2. The magnetic field sensing system of claim 1, whereinthe magnetic switching threshold of the MRAM cell corresponds to aminimum magnitude of a magnetic field incident upon the MRAM cell thatis required to change a magnetic state of the MRAM cell.
 3. The magneticfield sensing system of claim 1, further comprising a control moduleconfigured to: control one or more of the write current source and theswitching element to provide the write current from the write currentsource through the write line, such that the magnitude of the writemagnetic field is less than the magnetic switching threshold of the MRAMcell; after initiating the provision of the write current through thewrite line, determine a magnetic state of the MRAM cell; and determine apresence of an external magnetic field incident upon the MRAM cell basedat least in part on the determined magnetic state of the MRAM cell. 4.The magnetic field sensing system of claim 3, wherein the magnetic stateof the MRAM cell corresponds to a resistance of the MRAM cell, andwherein to determine the magnetic state of the MRAM cell, the controlmodule is configured to determine the resistance of the MRAM cell. 5.The magnetic field sensing system of claim 3, wherein the magnetic stateof the MRAM cell comprises a current magnetic state of the MRAM cell,and wherein the control module is further configured to: prior tocontrolling the one or more of the write current source and theswitching element to provide the write current, determine an initialmagnetic state of the MRAM cell, and wherein to determine the currentmagnetic state of the MRAM cell, the control module is configured todetermine a change in the initial magnetic state of the MRAM cell. 6.The magnetic field sensing system of claim 3, wherein the magnitudecomprises a first magnitude that is proportional to a second magnitudeof the write current, and wherein to control the one or more of thewrite current source and the switching element to provide the writecurrent and to determine the magnetic state of the MRAM cell, thecontrol module is configured to: control the one or more of the writecurrent source and the switching element to vary iteratively one or moreof the second magnitude of the write current so as to vary the firstmagnitude of the write magnetic field, and a polarity of the writecurrent so as to vary a polarity of the write magnetic field; and foreach iteration of varying the one or more of the second magnitude andthe polarity of the write current, determine a respective magnetic stateof the MRAM cell, and wherein to determine the presence of the externalmagnetic field incident upon the MRAM cell, the control module isconfigured to determine the presence of the external magnetic fieldbased at least in part on each of the determined magnetic states of theMRAM cell.
 7. The magnetic field sensing system of claim 3, wherein themagnitude comprises a first magnitude that is proportional to a secondmagnitude of the write current, and wherein the control module isfurther configured to determine a third magnitude of the externalmagnetic field based at least in part on the second magnitude of thewrite current and the magnetic switching threshold of the MRAM cell. 8.The magnetic field sensing system of claim 3, wherein the control moduleis further configured to determine a polarity of the external magneticfield based at least in part on a polarity of the write current.
 9. Themagnetic field sensing system of claim 1, wherein the MRAM cellcomprises a first MRAM cell, the magnetic switching threshold comprisesa first magnetic switching threshold, the write line comprises a firstwrite line, the write current comprises a first write current, the writemagnetic field comprises a first write magnetic field, the magnitudecomprises a first magnitude, and the switching element comprises a firstswitching element, the system further comprising: a second MRAM cellcomprising a second magnetic switching threshold; a second write lineassociated with the second MRAM cell and configured to conduct a secondwrite current so as to induce a second write magnetic field proximate tothe second MRAM cell, wherein the second write magnetic field has asecond magnitude that is less than the second magnetic switchingthreshold of the second MRAM cell; the write current source electricallycoupled to the second write line; and a second switching elementelectrically connected in series with the second write line andconfigured to provide the second write current from the write currentsource through the second write line.
 10. The magnetic field sensingsystem of claim 9, wherein the first magnetic switching threshold isdifferent than the second magnetic switching threshold.
 11. The magneticfield sensing system of claim 9, wherein the first write current isdifferent than the second write current.
 12. A method of sensing amagnetic field, the method comprising: providing a write current througha write line associated with a magnetoresistive random access memory(MRAM) cell so as to induce a write magnetic field proximate to the MRAMcell, wherein the write magnetic field has a magnitude that is less thana magnetic switching threshold of the MRAM cell; determining a magneticstate of the MRAM cell after initiating the provision of the writecurrent through the write line; and determining a presence of anexternal magnetic field incident upon the MRAM cell based at least inpart on the magnetic state of the MRAM cell.
 13. The method of claim 12,wherein the magnetic state of the MRAM cell corresponds to a resistanceof the MRAM cell, and wherein determining the magnetic state of the MRAMcell comprises determining the resistance of the MRAM cell.
 14. Themethod of claim 12, wherein the magnetic state of the MRAM cellcomprises a current magnetic state of the MRAM cell, the method furthercomprising: prior to providing the write current, determining an initialmagnetic state of the MRAM cell, and wherein determining the currentmagnetic state of the MRAM cell comprises determining a change in theinitial magnetic state of the MRAM cell.
 15. The method of claim 12,wherein the magnitude comprises a first magnitude that is proportionalto a second magnitude of the write current, and wherein providing thewrite current and determining the magnetic state of the MRAM cellcomprises: iteratively varying one or more of the second magnitude ofthe write current so as to vary the first magnitude of the writemagnetic field, and a polarity of the write current so as to vary apolarity of the write magnetic field; and for each iteration of varyingthe one or more of the second magnitude and the polarity of the writecurrent, determining a respective magnetic state of the MRAM cell, andwherein determining the presence of the external magnetic field incidentupon the MRAM cell comprises determining the presence of the externalmagnetic field based at least in part on each of the determined magneticstates of the MRAM cell.
 16. The method of claim 12, wherein themagnitude comprises a first magnitude that is proportional to a secondmagnitude of the write current, the method further comprisingdetermining a third magnitude of the external magnetic field based atleast in part on the second magnitude of the write current and themagnetic switching threshold of the MRAM cell.
 17. The method of claim12, further comprising determining a polarity of the external magneticfield based at least in part on a polarity of the write current.
 18. Themethod of claim 12, wherein the MRAM cell comprises a first MRAM cell,the magnetic switching threshold comprises a first magnetic switchingthreshold, the write line comprises a first write line, the writecurrent comprises a first write current, the write magnetic fieldcomprises a first write magnetic field, and the magnitude comprises afirst magnitude, the method further comprising: providing a second writecurrent through a second write line associated with a second MRAM cellso as to induce a second write magnetic field proximate to the secondMRAM cell, wherein the second write magnetic field has a secondmagnitude that is less than a second magnetic switching threshold of thesecond MRAM cell; after initiating the provision of the second writecurrent through the second write line, determining a magnetic state ofthe second MRAM cell; and determining a presence of an external magneticfield incident upon the second MRAM cell based at least in part on thedetermined magnetic state of the second MRAM cell.
 19. The method ofclaim 18, wherein the first magnetic switching threshold is differentthan the second magnetic switching threshold, the first write current isdifferent than the second write current, or both the first magneticswitching threshold is different than the second magnetic switchingthreshold and the first write current is different than the second writecurrent.
 20. A magnetic field sensing device comprising: means forproviding a write current through a write line associated with amagnetoresistive random access memory (MRAM) cell so as to induce awrite magnetic field proximate to the MRAM cell, wherein the writemagnetic field has a magnitude that is less than a magnetic switchingthreshold of the MRAM cell; means for determining a magnetic state ofthe MRAM cell after initiating the provision of the write currentthrough the write line; and means for determining a presence of anexternal magnetic field incident upon the MRAM cell based at least inpart on the magnetic state of the MRAM cell.